Radioactivity of Rocks oj the Kolar Gold-Fields. 47 



be quite sure that this was not due to faults of the method 

 or of the apparatus, a number of experiments were made. 



No increase in the leak was observed if the heating was 

 continued for more than 15 minutes. The resulting melt 

 nearly all went into solution on treatment with water, and 

 then hydrochloric acid. To be certain, however, that the 

 emanation had been expelled, these solutions, which were 

 quite clear, were kept for three weeks and then boiled under 

 reduced pressure and the gas introduced into the apparatus, 

 following the original method of Soddy (Roy. Soc. Proc. 

 lxxvi. 1905, p. 88). The small insoluble residue was also 

 brought into solution by fusion with carbonates, and the 

 solutions added to the others. Only a very slight increase in 

 the natural leak was ever observed, although a control expe- 

 riment with a solution to which 1 c.c. of the uranium nitrate 

 solution previously mentioned had been added, showed that 

 this method was not at fault, even though the results were 

 lower than those obtained by the other method. 



Although other observers have shown that very little 

 emanation is given up by a mineral on reducing the pressure 

 of the air in the vessel containing it, an experiment was 

 carried out in order to test this, in which only the electro- 

 scope was evacuated. No abnormal result was obtained. 

 The method of sweeping out the gas from the copper flask 

 was also found to remove at least 95 per cent, of the emanation; 

 and as all determinations were carried out in exactly the same 

 way, no error should arise from this cause. 



In all normal cases electroscope readings were taken 

 half hourly, for about 3 hours, starting about \\ hours after 

 admission of the gas, and the rates of leak reduced to their 

 maximum value by means of the curve given by latterly 

 (Phil. Mag. xx. 1910, p. 2). 



The decay curve after three to four hours corresponded 

 approximately with that of radium emanation, although an 

 accurate determination was difficult owing to the extremely 

 small quantities of gas. 



After a considerable number of determinations had been 

 made, an important source of error was discovered owing to 

 one specimen of rock giving rise to a leak which was less 

 than the normal leak of the electroscope. This was found to 

 be due to the presence of a large quantity of hydrogen in the 

 evolved gas. As the available data on ionization in hydrogen 

 are very meagre, some experiments were carried out to 

 determine the magnitude of the effect. It was found that 

 the natural leak in hydrogen was 1*0 scale-division an hour, 

 while in air it was G 0. Also in an experiment with uranium 



